1. Technical Field of the Invention
The present invention relates to an ink jet recording apparatus. More particularly, the present invention is directed to a technique for removing air bubbles existing in an ink passage formed in an ink jet recording apparatus which is arranged to discharge ink droplets through openings of a nozzle thereof.
2. Background of the Related Art
A conventional ink jet recording apparatus comprises a recording head structured in such a manner that an actuator, including a piezoelectric vibrator or heating device or the like, applies pressure to ink in a pressure generating chamber to discharge ink droplets through openings formed in a nozzle thereof. Thus, ink in an opening in the nozzle, from which ink droplets are not discharged in a large quantity during the printing operation, is not renewed. Thus, the viscosity of ink in the opening portion in the nozzle is undesirably raised As a result, the quality of the printed product deteriorates.
To solve the above-mentioned problem, a flushing operation has been performed whenever the printing operation is performed for a predetermined time, e.g., 20 seconds. Thus, the recording head is moved to an ink receiver of a capping unit disposed at a standby position in a non-printing region. Moreover, ink droplets are discharged through the openings in the nozzle regardless of data which must be printed. Another operation has been performed in such a manner that the openings in the nozzle are sealed by the capping unit, and then negative pressure is applied to forcibly discharge ink through the openings in the nozzle.
The operation for forcibly discharging ink through the openings in the nozzle enables ink having raised viscosity and existing near the openings in the nozzle to be removed reliably. Moreover, the foregoing operation enables any air bubble existing in the ink passage in, for example, the recording head, to be removed.
The ink jet recording head which uses a piezoelectric vibrator to serve as the actuator is structured in such a manner that ink droplets are discharged by mechanically changing the capacity of the pressure generating chamber by using displacement of the piezoelectric vibrator. Therefore, controlling the level of a signal which is supplied to the piezoelectric vibrator enables the quantity of ink forming the ink droplet to be adjusted precisely. Thus, a recording apparatus capable of printing an image which must be formed by dense dots can be structured.
To cause a slight displacement of the piezoelectric vibrator to correspond to the quantity of ink forming an ink droplet, the fluid characteristics of the ink existing in the recording head, and more particularly constant compressibility and elastic modulus of the ink, must be maintained. If a large quantity of air is dissolved in the fluid ink, the apparent compressibility of the ink is raised. In this case, the displacement of the piezoelectric vibrator cannot be made to correspond to the change in the pressure of the ink. As a result, the ink discharge characteristic is changed undesirably. To prevent the above-mentioned problem, deaeration of the ink accommodated in the ink cartridge and accommodation of the ink cartridge in an air shielding container to prevent increase of the quantity of dissolved air have been performed.
When the ink cartridge has been mounted on the recording apparatus, air is introduced through an opening which communicates with the atmosphere and which permits ink to be introduced into the recording head. Thus, introduced air is dissolved in ink. The quantity of air dissolved in the ink increases over time. Thus, change in the pressure occurring in the pressure generating chamber for discharging ink droplets causes cavitation to take place. As a result, the reliability of a formed ink droplet deteriorates as compared with that of an ink droplet formed by the ink in an ink cartridge immediately after being discharged from an air shielding container.
When air bubbles, which are generated in the pressure generating chamber and fluid passage after the apparatus has been used for a long time, are removed, ink in the ink cartridge is forcibly substituted for ink existing in the pressure generating chamber if a small quantity of air is dissolved in the ink in the ink cartridge. Thus, air can be dissolved in the ink and the air can easily be removed. However, if the quantity of air dissolved in the ink in the ink cartridge is increased, the air bubbles cannot be dissolved in ink. In this case, there arises a problem in that a large quantity of ink is required to remove the air bubbles because the structure is arranged in such a manner that the air is discharged through the nozzle along the flow of the ink.